(1) Field of the Invention
The present invention relates to a magnetic recording medium comprising an aromatic polyamide film and a magnetic layer vacuum-deposited on the aromatic polyamide film, which is excellent in adhesive strength and flatness of the magnetic layer and also in the balance of environmental stability in dimension.
(2) Description of the Prior Art
Aromatic polyamide films have heretofore been used at high temperatures by virtue of their high heat resistance, and uses of laminates of aromatic polyamide films with other materials under high temperature conditions have been examined. When an aromatic polyamide is used alone in the form of a single-layer film, it is preferred from the viewpoint of flatness or stabilization of shape that the thermal dimensional change be as small as possible. However, when an aromatic polyamide is used in the form of a laminate with a metal or other material of different thermal dimensional change characteristic, wrinkles or curls take place during use of the laminate at a high temperature or after preparation of the laminate at a high temperature. As the result, the flatness is reduced. Although "thermal dimensional change" includes thermal shrinkage and thermal expansion, conventional single-layer films, however, have been prepared while taking only one aspect of the thermal dimensional change into consideration.
Recently, a demand has arisen for increased recording density in magnetic recording media and means for attaining such an increase have become a focus of attention in the art. The conventional magnetic recording medium of the so-called coating type was formed by coating a homogeneous dispersion of a magnetizable powder of a metal oxide such as .gamma.-Fe.sub.2 O.sub.3 or a cobalt or nickel alloy in a binder of an appropriate organic polymer on a plastic base film. Recently, a magnetic recording medium has been developed which is prepared by forming a high performance thin magnetic metal layer of cobalt or the like, free of a polymer binder, directly on a base by vacuum deposition or sputtering. By the term "vacuum deposition" is meant a process in which a metal or a metal compound or a mixture thereof is sublimated under a lower pressure than normal pressure and deposited on a base film.
Serious problems, however, have been encountered in the practical utilization of a magnetic recording medium having such a high performance thin magnetic metal layer. For example, there is poor adhesive strength between the base and the high performance thin magnetic metal layer and also inferior flatness due to the base film deformation and the difference of the thermal expansion coefficient between the base and the high performance thin magnetic metal layer. The high performance thin magnetic metal layer ordinarily has a thickness of 200 to 7,000 .ANG., for example, about 1,000 .ANG.. Since the thickness of the magnetic layer is extremely thin, an especially high surface smoothness is required for the base which has a crucial effect on the magnetic layer surface, while such a high surface smoothness is not required in the magnetic recording medium of the conventional coating type at all, where the magnetic layer is usually thicker than 2 .mu.m. This makes impossible the use of the method of roughening the surface of the base to improve the adhesive strength. Furthermore, since the high performance thin magnetic metal layer is composed solely of a metal or metal compound, it is poor in elongation, in other words, easily breaks at small elongation and, since no buffer effect as attained in the magnetic recording medium of the coating type can be expected, the resistance to external force is very low. Moreover, the fact that the base is ordinarily composed of an organic polymer poses the problem of bonding organic and inorganic materials, a problem which may be overlooked in the magnetic recording medium of the coating type where the compatibility of the coating layer with the base is enhanced by the use of the organic binder.
Various methods have been proposed to improve the adhesive strength of the high performance thin magnetic metal layer. For example, in case of vacuum deposition, there has been proposed a method in which electron beam vacuum deposition conditions are changed so as to obtain optimum results and a method in which a polyethylene terephthalate film, which is most popularly used as the base, is subjected to a surface treatment such as a glow discharge treatment or buff treatment. However, no substantial improvement of adhesive strength can be attained by these methods. First, regardless of the results of adhesive cellophane tape tests, these methods do not result in adhesive strength satisfactory for practical use. In practical use, "adhesive strength" has a broader meaning, i.e., the high performance thin magnetic metal layer may not peel or be damaged when the magnetic recording medium, especially magnetic tape, is brought into contact with a head or a roll of the tape running system. In the case of a video tape, the magnetic recording medium is exposed to very high relative speed contact with a head, repeated contact in the same portion at the still position, contact with a post, and other severe conditions. In practice, therefore, the adhesive strength must be so high that there is no substantial drop-out even after the magnetic recording medium is subjected to more than 100 times of running. Second, the stiffness and toughness of the high performance thin magnetic metal layer are important for attaining an adhesive strength satisfactory in the above-mentioned practical sense, and even if the surface treatment of the base is sufficiently performed, the characteristics of the deposited material will be improved only to a minor extent. Third, as pointed out hereinbefore, the physical surface treatment has a fatal adverse effect on the surface smoothness of the base of the magnetic recording medium comprising the high performance thin magnetic metal layer formed on said base.
In order to obtain an excellent vacuum deposited magnetic recording medium, the following endurance requirements and the above-mentioned adhesive strength requirements should be met.
(1) High mechanical strength characteristics. Not only the strength and elongation, but also the tensile modulus should be high. In other words, the dimensional stability against external force should be high.
(2) Excellent dimensional stability at high temperatures.
(3) High dimensional stability against ambient changes such as moisture and temperature.
In the preparation of a magnetic recording medium by vacuum deposition, the temperature of the base film is readily elevated, and thus, vacuum deposition is carried out at an elevated temperature. Elevation of the temperature of the base film is advantageous for improving the magnetic characteristics and adhesive strength, but necessitates a high heat resistance for the base film. Further, in order to prevent degradation of the flatness of the base film owing to the different thermal dimensional characteristics of the material to be deposited, it is important that the base film show a specific thermal dimensional change at the elevated temperature at the time of vacuum deposition. An aromatic polyamide film exhibiting a reduced dimensional change and a magnetic recording medium comprising this film are proposed in U.S. Pat. Nos. 3,966,686 and No. 4,112,187. According to these known techniques, reduction in the dimensional change of the base film is intended. However, the magnetic recording medium prepared by vacuum deposition using this base film has some problems in the flatness and other properties.